Commercially available articulated vehicles, such as forest vehicles and loading machines, are often equipped with conventional mechanical and/or hydraulic power trains. In such vehicles the force from an engine is transmitted to the drive wheels through an arrangement of shafts, hydraulics and gears. Such arrangements are often very complicated, heavy and expensive to manufacture, they further require extensive maintenance. Moreover a differential gear has to be arranged between each pair of drive wheels to make it possible for the vehicle to turn. However, in order to achieve acceptable availability, e.g. in difficult terrain, the power train has to be equipped with a differential lock/break arrangement that counteracts the differential gear, e.g. if one of the drive wheels loose traction. Such lock/break arrangements make the mechanical power train even more complicated, at the same time as the maneuverability is vastly deteriorated when they are activated. Moreover the complex power train gives rise to considerable power losses, that in turn gives rise to higher fuel consumption.
The drawbacks of mechanical power trains are especially obvious for vehicles with more than two pair of drive wheels, such as forest vehicles with three or more pairs of drive wheels. Tests have been performed to replace the mechanical power train with an essentially fully hydraulic power train where each drive wheel is driven by an individual hydraulic drive motor. However, these trials have shown that power trains require a very complicated control system in order to achieve the differential function from a conventional mechanical power train, as the system may encounter a pressure drop if one wheel looses traction (looses the contact with the ground).
Different types of electrically propulsed vehicles have been proposed during the years, but for different reasons only a few have been commercially successful. Electrical propulsion is something that so far almost exclusively has been related to small vehicles up to the size of an automobile. U.S. Pat. No. 3,171,505 discloses an automobile with individual electrical propulsion of the drive wheels of the automobile.
JP 2002-010405 discloses an electrical power train for an articulated loading machine, truck or the like, wherein each wheel is driven by an individual electrical motor and the frequency of rotation for each wheel during a turning operation is controlled by a control unit, and wherein the conventional direct steering system is omitted and replaced by individual control of the rotational frequency for each wheel in response to a steering-angle sensor. In order to perform a steering operation, the control unit calculates the correct frequency of rotation for each wheel with respect to the drive wheel angle, the velocity and the time point for changing the angle of the drive wheel. Hence, the steering operations for the vehicle are based on advanced algorithms, and the steering system can be characterized as being indirect as it does not comprise any means for direct actuation of steering operations. However, in order for this system to work, the ground must be essentially flat and where no skidding occurs, whereby such a system is practically useless in many situations where the ground is non-flat, e.g. for forest vehicles, which by definition operates in rough terrain where wheels regularly skids.